Crystal structure change in the dehydrogenation process of the Li-Mg-N-H system

Abstract The Li–Mg–N–H system has the property of reversible reaction with hydrogen between hydrogenation and dehydrogenation (Mg 3 N 2  + 4Li 3 N + 12H 2  ↔ 3Mg(NH 2 ) 2  + 12LiH). At the several dehydrogenation stages of p–c isotherm measurement at 523 K, the structural change was investigated using the synchrotron X-ray diffraction. There are two regions in p–c isotherm of the Li–Mg–N–H system, i.e. plateau and sloping region. In the plateau region, Mg(NH 2 ) 2 and Li 3 Mg 3 (NH 2 )(NH) 4 coexist. In the sloping region, the intermediate phase Li 3+3 y Mg 3 (NH 2 ) 1− y (NH) 4+2 y changes continuously from Li 3 Mg 3 (NH 2 )(NH) 4 to Li 2 Mg(NH) 2 . The chemical composition of the intermediate phase was estimated from the amount of desorbed hydrogen by p–c isotherm and the atomic ratio of Mg and N by Rietveld analysis. The crystal structure of the intermediate phase, Li 3+3 y Mg 3 (NH 2 ) 1− y (NH) 4+2 y (space group: I 222), was determined. Because all these intermediate structures are similar to anti-CaF 2 -type, it is deduced that the dehydrogenation process are caused by the diffusion of Li + to cation sites of Mg(NH 2 ) 2 . The analysis of structural change clarified the dehydrogenation process that is accomplished by the diffusion of Li + and Mg 2+ without N atom diffusion.